A conventional automotive safety glazing is formed from a laminate made of two rigid layers, typically glass, and an anti-lacerative mechanical energy absorbing interlayer of plasticized polyvinyl butyral (PVB). The glazing is prepared by placing the PVB layer between glass sheets, eliminating air from the engaging surfaces, and then subjecting the assembly to elevated temperature and pressure in an autoclave to fusion bond the PVB and glass into an optically clear structure. The glazing may then be used in the windows, including those in the front (windshield), the rear, and the side glass of a motor vehicle.
The laminate may also include at least one functional layer engineered to enhance the performance of the vehicle window. One important functional layer reduces entry of infrared radiation into the vehicle cabin. When used in a windshield, the composite laminate structure should transmit at least about 70% of the light in the wavelength region sensitive to the human eye, typically from about 400 to about 700 nanometers (nm), and reject solar radiation outside the visible portion of the spectrum. When used in other glazing structures, such as side or rear windows, there are typically no limits on the level of visible transmission.
The functional layer in the laminate may be a birefringent, non-metallic film made from alternating layers of dielectric materials, preferably polymers with differing indices of refraction. These birefringent films may be engineered to reflect or absorb a desired amount of light in a spectral region of interest (such as the infrared region) while transmitting sufficient visible light in the visible region of the spectrum to be substantially transparent.
The reflectance characteristics of the multilayer film are determined in part by the indices of refraction for the layered structure. In particular, reflectivity depends upon the relationship between the indices of refraction of each material in the x, y, and z directions (nx, ny, nz). The film is preferably constructed using at least one uniaxially birefringent material, in which two indices (typically along the x and y axes, or nx and ny) are approximately equal, and different from the third index (typically along the z axis, or nz). The x and y axes are defined as the in-plane axes, in that they lie in the plane within the multilayer film, and the respective indices nx and ny are referred to as the in-plane indices. If n1z is selected to match n2x=n2y=n2z and the multilayer film is biaxially oriented, there is no Brewster's angle for p-polarized light and each interface exhibits constant reflectivity for p-polarized light for all angles of incidence.
A second factor that influences the reflectance characteristics of the multilayer film is the thickness of the layers in the film stack. The individual layers are arranged in groups of two or more that repeat throughout the stack, referred to as optical repeat units or unit cells, each of which has a total optical thickness that is ½ of the wavelength of light to be reflected. All thicknesses discussed herein are measured after any orientation or other processing, unless otherwise noted. The term optical thickness refers to the physical thickness multiplied by the refractive index, which may be a function of polarization (for birefringent materials) and wavelength (for dispersive materials).
The infrared (IR) reflecting films described in U.S. Pat. No. 5,882,774 (Jonza et al.) and U.S. Pat. No. 6,049,419 (Wheatley et al.) control the amount of solar energy that pass through them, preferably without significantly decreasing the intensity or changing the color of light sensed by the human eye at any angle. The materials in the layers, the thicknesses of the layers, and the indices of refraction of the layers are selected to reflect infrared radiation within the wavelength range of about 700 nm to about 2000 nm, while transmitting visible light. The film has an average reflectivity of at least 50% over a band at least 100 nm wide in the infrared region of the spectrum.
In one design, the IR reflecting film may include a multilayer stack of unit cells composed of alternating layers of first (A) and second (B) polymers, usually with similar optical thicknesses, referred to herein as an AB construction.
In an alternative design, the IR reflecting film described in U.S. Pat. No. 5,360,659 (Arends et al.) may also have multilayer stack of unit cells composed of alternating layers of first (A) and second (B) polymers. In this construction the unit cells have six layers with relative optical thicknesses of about .778A.111B.111A.778B.111A.111B. This construction, referred to herein as the 711 construction, suppresses unwanted second, third, and fourth order reflections in the visible wavelength region of between about 400 to about 700 nm, while reflecting light in the infrared wavelength region of between about 700 to about 2000 nm. Reflections higher than fourth order will generally be in the ultraviolet, not visible, region of the spectrum or will be of such a low intensity as to be unobjectionable.
Reference is also made to U.S. application Ser. No. 09/590,924 (Liu et al.), now U.S. Pat. No. 6,797,396, priority document to PCT Publication No. WO 01/96104, which discusses multilayer optical films useful in laminating to substrates having a compound curvature, such as motor vehicle windshields.
To reflect over a wide band, the unit cells in either of the film designs described above preferably have varying optical thicknesses, referred to herein as a layer thickness gradient, which are selected to achieve the desired bandwidth of reflection. The layer thickness gradient may vary widely depending on the intended application for the film. For example, the layer thickness gradient may be linear, in which the optical thickness of the unit cells (and each of their component layers) increases at a constant rate across the thickness of the film. In this construction, each unit cell is a certain amount thicker than the thickness of the previous unit cell in the multilayer stack. The layer thickness may decrease, then increase, then decrease again from one major surface of the film to the other, or may have an alternate layer thickness distribution designed to increase the sharpness of one or both bandedges, as described in U.S. Pat. No. 6,157,490 (Wheatley et al.).
The multilayer IR reflecting film designs described above have very high visible light transmission, and are useful as IR mirrors or IR polarizers in automotive glazing laminates. When used in a windshield laminate construction, these IR mirrors and polarizers have low reflection in the visible region (referred to as veiling glare in the automotive arts), which enhances performance. However, the IR mirrors and polarizers may also generate unwanted colors (iridescence) in certain laminate constructions, which may be unacceptable in demanding automotive and architectural applications.